Three-dimensional (3D) displays have attracted great attention over the past five decades. 3D virtual objects were originally displayed with a head-mounted display. Since then, continuous efforts have been made to explore 3D displays that have planar surfaces, and several methods have been developed to provide stereopsis for binocular vision. The 3D displays that employ glasses to achieve this are based on techniques such as anaglyphs, time-division, and polarization. On the other hand, those 3D displays that do not rely on glasses are based on techniques such as parallax barrier and lenticular lens array. Although these methods can offer effective 3D images, they require calculation and generation of precise images for multiple viewpoints, and users have to stay within a limited view angle.
A different approach to realizing advanced 3D displays is using a physical 3D space to render graphics instead of a planar surface. This approach forms a visual representation of an object in three physical dimensions, as opposed to the planar image of traditional screens that simulate depth through a number of different visual effects. These 3D displays, which are called volumetric displays, allow users to view the displayed images from any angle. Volumetric displays arranged “voxels” in a 3D space.
Lasers can be used to induce light spots (“voxels”) in various media at arbitrary points in three dimensional space. Some of the advantages of laser-induced light spots include: (1) no need to arrange special materials and suspend it in a medium to emit light; (2) wireless transmission of power so that structures that possibly obstruct the line-of-sight can be avoided; and (3) precise control of the laser owing to the progress in optical technologies. In an aerial laser-based volumetric display, voxels in air, i.e., plasma, are generated by high-intensity lasers which are achieved by shortening pulse duration (e.g., nanoseconds) under a limited total power.
An aerial volumetric display was first demonstrated using a nanosecond laser where a rendering speed of only 100 dot/sec was achieved. (Kimura et al. 2006). Later, 1,000 dot/sec was achieved by using a femtosecond laser. (Saito et al. 2008). However, these systems had low resolution.
Laser-based volumetric displays in media other than air have also been demonstrated. A nanosecond laser-based volumetric display in water was developed where a rendering speed of 50,000 dot/sec was achieved. (Kimura et al 2011). Later, a femtosecond laser-based volumetric display in a fluorescent medium with parallel optical access via computer generated hologram was developed to achieve higher resolution. (Hasegawa et al. 2013). In these systems, the induced light spots are not accessible.
Aerial volumetric displays are usually accompanied by interaction with a user's hand. It would be advantageous to integrate aerial haptics into an aerial volumetric display in order to provide tactile sensation to a user interacting with virtual objects. Aerial haptics is advantageous because force can be projected from a distance without physical contact or wearable devices, and it has high programmability. In other words, it can be set and rearranged at an arbitrary position in a 3D space because it does not require physical actuators. For example, recent research has explored the use of an array of ultrasonic transducers (Hoshi et al. 2010; Carter et al. 2013; Inoue et al. 2014) or air vortices (Sodhi et al. 2013; Gupta et al. 2013) for non-contact haptic stimulation and feedback. These approaches lack spatial precision and has limited working distance. Recent research has also explored the use of a low power nanosecond laser to evoke tactile sensation on human skin. (Jun et al. 2015; Lee et al. 2015). However, it was shown that even a low power nanosecond laser can instantaneously increase the temperature of human skin.
Aerial volumetric displays are usually accompanied by a sound system. Conventional studies on controlling spatial sound distribution in free space include multi-channel audio synthesis and ultrasound based superdirective speakers (parametric speakers) as a means of generating 3D acoustics. Conventional surround sound speakers simulate an immersive sonic environment by using multiple speakers to surround a target area and generate a spatial pattern via interference of audible sound waves (Shinagawa et al. 2007). But the quality of the aural experience depends on the position of the listener relative to the placement of the surround sound speakers and the aural experience is generally optimized for a listener located at the center of the target area. Parametric speakers can generate a narrow beam of audible sound using an ultrasonic transducer array, such that only individuals targeted by the device is able to hear the emitted sound. (Yoneyama et al 1983). In both of these sound systems, audible sound is generated from outside the target area and emitted towards the target area.
Consequently, it would be desirable to have a high resolution and scalable aerial volumetric display. It would also be desirable to have an aerial volumetric display with improved non-contact haptic feedback. It would also be desirable to have a personalized immersive spatial audio experience to accompany a 3D visual experience viewed from a particular vantage point.